1. Field of the Invention
The present invention involves flux cored arc welding and specifically to improved processing parameters resulting from the employment of specific shielding gas combinations. These combinations have been selected to significantly reduce fume emission levels while providing equivalent or better welding performance at normal operating parameters.
2. Description of the Background
Flux cored arc welding is a process that uses an arc between a continuous filler metal electrode and weld pool. The process is used with shielding from a flux contained within the tubular electrode, with or without additional shielding from an externally supplied gas, and without the application of pressure. Flux cored arc welding generally employs two major process variations for shielding the arc and weld pool from atmospheric contamination. A self-shielded embodiment protects the molten metal through the decomposition and vaporization of the flux core by the heat of the arc. The second embodiment involves a protective gas flow in addition to the flux core. With both embodiments, the electrode core material provides a substantial slag covering to protect the solidifying weld metal.
Typically, gas shielding requires a gas source, a pressure regulator, a flow metering device and necessary hoses and connectors. Shielding gases are dispensed from cylinders, manifolded cylinder groups or from bulk tanks which are piped to individual welding stations. Regulators and flow meters are used to control pressure and flow rates. Since regulators can freeze during rapid withdrawal of gas from storage tanks, heaters are available to prevent that complication. Welding grade gas purity is required because small amounts of moisture can result in porosity or hydrogen absorption in the weld metal. The dew point of shielding gases should be below -40.degree. F. Suitable sources of such gases is shown in U.S. Pat. No. 4,973,822 dated Nov. 27, 1990, the disclosure of which is incorporated herein by reference.
Carbon dioxide is widely used as a shielding gas for flux cored arc welding. Two advantages of this gas are its low cost and deep weld penetration. It is also known to use gas mixtures such as the combination of an inert gas with carbon dioxide or oxygen. The "rule of thumb" is that the higher the percentage of inert gas in mixtures with carbon dioxide, the higher will be the transfer efficiencies of the deoxidizers contained in the core. The presence of an inert gas, such as argon, in sufficient quantities in a shielding gas mixture results in less oxidation than occurs with 100% carbon dioxide shielding.
The most common gas mixture for flux cored arc welding is 25% carbon dioxide and 75% argon (by volume). It has been observed that weld metal deposited with this mixture generally has higher tensile and yield strengths than weld metal deposited with 100% carbon dioxide shielding.
A good bit of attention has been directed toward the study and development of flux cored arc welding consumables which exhibit low fume emission rates. This is achieved by using flux components in the core of the wire that generate lower quantities of fumes upon degeneration in the welding arc. This research has produced consumables that have recently appeared on the market.
Little development work has been focused in the area of producing a shielding gas which provides decreased fume emission levels using all types of flux cored arc welding consumables. A shielding gas developed for process applications while promoting lower fume emission levels must also provide equivalent or better welding performance, at normal operating parameters, as compared to conventional flux cored arc welding shielding gases.
It is thus an object of the present invention to provide a shielding gas mixture that promotes significantly lower fume emission levels compared to that of commercially pure carbon dioxide and 75% argon/25% carbon dioxide mixtures as shielding gases while providing equivalent or better welding performance at normal operating parameters.
This and further objects will be more readily apparent when viewing the following disclosure and appended claims.